Method for enhancing the properties of biofouling release coatings

ABSTRACT

The present invention relates to a method of introducing enhanced biofouling release properties to intact biofouling release coatings by exposing the surface of the biofouling release coating to a restorative compound for a time sufficient to effect enhancement of biofouling release properties. Also disclosed are kits useful in carrying out these processes.

BACKGROUND OF THE INVENTION

This invention was made with Government support under GovernmentContract No. N00014-96-C-0145 awarded by DARPA. The Government may havecertain rights to the invention.

This invention is related to the field of biofouling release coatingsfor use in industrial, commercial or military marine and freshwaterapplications. In particular, this invention relates to a method forregenerating a biofouling release coating which has decreased releaseefficacy due to depletion or lack of an incorporated oil.

Damage to underwater power cables, ships, and the like due tocolonization of their surfaces by organisms (including, but not limitedto, barnacles) has serious economic consequences in marine andfreshwater industries. Antifouling and fouling release coatings havebeen developed to prevent or reduce biofouling, and to loosen thestrength of the attachment of marine organisms to make cleaning surfaceseasier. There are many commercial foul release coatings including, forexample, GE EXSIL® 2200. However there have been no reports of renewalmethods for these coatings.

There has been a continuing need in the coatings industry for newmethods for increasing the useful life of fouling release andantifouling coatings. At present, the useful lifetime of a copperablative antifouling coating is approximately three years, after whichtime the coating must be removed from the hull and reapplied. It isestimated that the effective life span of silicone fouling releasecoatings is about 5-7 years.

The release characteristics of silicone fouling release coatings areknown to be significantly enhanced by the addition of oils such asmineral oil and silicone oils. Barnacle adhesion measurements on foulingrelease coatings substantiate that removal of fouling requires less workwhen the silicone topcoat has been prepared with incorporated oils. Forexample, silicone oils such as dimethyl silicone oils, phenyl-modifiedsilicone oils, and polyether-modified silicone oils have beenincorporated into biofouling release coatings.

Unfortunately, these additives tend to diffuse out of the coating duringuse and are thus rapidly depleted. The depleted coatings lose theirenhanced foul-release properties, and consequently their effectivenessis reduced. Depletion of the additive therefore limits the useful lifeof the coating, necessitating periodic removal and reapplication of anew silicone biofouling release coating. A recoat technology for thesecoatings which does not require complete removal and reapplication ofthe coating would significantly reduce life- cycle costs and enhance theattractiveness of these coatings to the power utility, military,industrial, and commercial markets.

To forestall the rapid oil-depletion of oil-containing biofoulingrelease coatings, larger amounts of oil have been incorporated intothese coatings. This solution to the problem does curtail the rapiddepletion of the oil, but unfortunately it tends to severely impair themechanical properties of the coatings, particularly tear strength andabrasion resistance. Increasing the original additive content ofbiofouling release coatings, therefore, does not provide a workablemethod of increasing the life of the coatings.

Silicone biofouling release coatings made without additives also havebeen used to make cleaning surfaces of organisms easier, but they arenot as effective as coatings with incorporated oils. A method forenhancing the properties of these coatings, as well as restoring theenhanced release properties of older coatings originally containingadditives is highly desirable.

SUMMARY OF THE INVENTION

The present invention relates to a method of introducing enhancedbiofouling release properties to an intact biofouling release coating,which method comprises exposing the surface of the biofouling releasecoating to a restorative compound for a time sufficient to effectenhancement of biofouling release properties.

In a further aspect, the present invention relates to a kit for theenhancement of the biofouling release properties of an intact biofoulingrelease coating, said kit comprising a container containing arestorative compound suitable for said enhancement.

In yet another aspect, the present invention relates to a kit for theenhancement of the biofouling release properties of an intact biofoulingrelease coating, said kit comprising a container containing a biofoulingrelease coating, and a container containing a restorative compoundsuitable for said enhancement.

DETAILED DESCRIPTION OF THE INVENTION

The term "oil-depleted" referring to release coatings will be used inthis application to denote any biofouling release coating, whethermanufactured with or without incorporated oils, which has been depletedof the oil additive or lacks the oil additive, and therefore has areduced effectiveness compared to coatings containing the oil additive.It is not to be construed as limited solely to coatings which weremanufactured containing oil and have subsequently been depleted of theoil.

The biofouling release coatings which may be enhanced by the presentinvention include generally any coating into which a restorativecompound may be incorporated for enhancement of biofouling releaseproperties. The present invention is particularly applicable to releasecoatings which include a conventional one-part or two-part RTVcomposition, preferably a two-part composition. It may comprise at leastone reactive silicone, at least one condensation catalyst and at leastone crosslinking agent.

The reactive silicone is preferably at least one of apolydialkylsiloxane, a polydiarylsiloxane, or a polyalkylarylsiloxanetypically of the formula ##STR1## wherein each R¹ is a hydroxyl radicalor ##STR2## each R² is independently a hydrocarbon or fluorinatedhydrocarbon radical, each R³ and R⁴ is independently a hydrocarbonradical, a is 0 or 1, and m has a value such that the viscosity of saidcompound under ambient temperature and pressure conditions is up toabout 50,000 centipoise. Illustrative hydrocarbon radicals are C₁₋₂₀alkyl, C₆₋₂₀ aryl and alkaryl, vinyl, isopropenyl, allyl, butenyl andhexenyl, with phenyl, C₁₋₄ alkyl and especially methyl being preferred.An illustrative fluorinated hydrocarbon radical is3,3,3-trifluoropropyl. Preferably, each R², R³ and R⁴ is alkyl andpreferably methyl. The biofouling release coatings may comprise two ormore reactive silicones, differing in average molecular weight, whichmay afford a bimodal composition having performance advantages over asimple monomodal composition.

The condensation catalyst may be any of those known to be useful forpromoting condensation curing of an RTV material. Suitable catalystsinclude tin, zirconium, titanium, and aluminum compounds as illustratedby dibutyltin dilaurate, dibutyltin diacetate, dibutyltin methoxide,dibutyltin bis(acetylacetonate), 1,3-dioxypropanetitaniumbis(acetylacetonate), titanium naphthenate, tetrabutyl titanate,zirconium octanoate, and aluminum acetylacetonate. Various salts oforganic acids with such metals as lead, iron, cobalt, manganese, zinc,antimony and bismuth may also be employed. For most purposes, the tinand titanium compounds are preferred.

As crosslinking agents, trifunctional (T) and tetrafunctional (Q)silanes are useful, the term "functional" in this context denoting thepresence of a silicon-oxygen bond. They include such compounds asmethytrimethoxysilane, methyltriethoxysilane,2-cyanoethyltrimethoxysilane, methyltriacetoxysilane, tetraethylsilicate and tetra-n-propyl silicate. The Q-functional compounds, i.e.,tetraalkyl silicates, are often preferred.

The coating may contain other constituents, including reinforcing andextending (non-reinforcing) fillers. Suitable reinforcing fillers arecommercially available in the form of relatively large aggregatedparticles typically having an average size significantly greater thanabout 300 nanometers (nm). The preferred fillers are the silica fillers,including fumed silica and precipitated silica. Those two forms ofsilica have surface areas in the ranges of 90-325 and 8-150 m² /g,respectively.

The reinforcing filler is most often pretreated with a treating agent torender it hydrophobic. Typical treating agents include cyclic siliconessuch as cyclooctamethyltetrasiloxane and acyclic and cyclicorganosilazanes such as hexamethyldisilazane,1,3-divinyl-1,1,3,3-tetramethyldisilazane, hexamethylcyclotrisilazane,octamethylcyclotetrasilazane, and mixtures thereof. Hexamethyldisilazaneis often preferred.

Non-reinforcing fillers include titanium dioxide, lithopone, zinc oxide,zirconium silicate, iron oxides, diatomaceous earth, calcium carbonate,glass fibers or spheres, magnesium oxide, chromic oxide, zirconiumoxide, aluminum oxide, crushed quartz, calcined clay, talc, kaolin,asbestos, carbon, graphite, cork, cotton and synthetic fibers.

The proportions of the constituents of the silicone component may bevaried widely. The amount of filler is generally about 5-200 parts andpreferably about 10-150 parts by weight per 100 parts of reactivesilicone. Catalysts and crosslinkers are generally present in theamounts of about 0.001-2.5% and about 0.25-5.0% by weight respectively,based on the combination of reactive silicone and filler.

Restorative compounds which may be used in connection with the presentinvention include oils such as polyorganosiloxanes (e.g.,polyalkylsiloxanes, organic-compatible siloxanes,polymethylphenylsiloxanes, polydiphenylsiloxanes, hydrophilic siloxanes,carbinolfunctional siloxanes, and related compounds); crude oil products(e.g., paraffin wax, petroleum waxes, petrolatum, liquid paraffin, andgreases); and fats, oils and waxes.

Oil-depleted biofouling release coatings may be restored by applying therestorative compound to the release coating surface. Such applicationmay be by soaking, dipping, spraying, wiping, brushing, coating orotherwise exposing the coating surface to the desired restorativecompound. During the application process, it is desirable to maintainthe restorative compound in contact with the coating for a period oftime sufficient to ensure adequate uptake of the restorative compound bythe coating. The optimum period will vary according to a number offactors, including the identity and condition of the coating, theidentity of the restorative compound, etc. The best contact time periodfor a given set of conditions may be readily determined by one ofordinary skill. A preferred period is at least about 10 hours, morepreferably from about 10 to about 90 hours, and most preferably fromabout 24 to about 72 hours. Sufficient volumes of restorative compoundto completely cover or immerse the surface of the coating are desirable,but not necessary. After treatment, the surface may be wiped dry, ifdesired. Any excess restorative compound may be recovered from the wipefor reuse by means known in the art.

The restorative compound useful in the present invention may be sold inthe form of a kit, i.e., in a suitable container (e.g., a drum, can,carton, etc.), optionally with instructions for use being present in thekit, for example attached to or in association with the container. Thekit may also comprise a container having a biofouling release coating,preferably compatible with the restorative compound.

The invention will be illustrated by the following nonlimiting Examples.

EXAMPLES

1. PREPARATION OF BIOFOULING RELEASE COATING

An aluminum platen (4 in.×12 in.) was primed by wiping the surface withthe commercially available primer SS4155 (GE Silicones). A coating wasthen prepared from 100 grams (g) of a silicone RTV composition (approx.71% polydimethylsiloxane [viscosity, 310 centipoise], 29% calciumcarbonate, and 2% ethyl silicate), catalyzed with 0.5 g dibutyltindilaurate, and applied to the platen to a dry film thickness of 15 milsusing a draw down blade. The coating was allowed to cure at roomtemperature for 7 days.

2. PREPARATION OF EXTRACTED (AGED) BIOFOULING RELEASE COATING

A biofouling release coating prepared as in Example 1 was extractedseven times in 200 milliliters (ml) toluene over a period of 3 days anddried.

3. PREPARATION OF REGENERATED BIOFOULING RELEASE COATING

An extracted biofouling release coating prepared as in Example 2 wassoaked in a bath of approximately 300 ml polydimethyldiphenylsiloxaneoil (number average molecular weight [Mn] about 2000) for 3 days, andthen wiped dry.

4. PREPARATION OF ADDITIVE-ENHANCED BIOFOULING RELEASE COATING

An aluminum platen primed as in Example 1 was coated with a biofoulingrelease coating prepared from 100 g of a silicone RTV composition(described in example 1), 10 g polydimethyldiphenylsiloxane oil (Mnabout 2000), and 0.5 g dibutyltin dilaurate, to a dry film thickness of15 mils using a draw down blade. The coating was allowed to cure at roomtemperature for 7 days.

5. PREPARATION OF EXTRACTED (AGED) ADDITIVE-ENHANCED BIOFOULING RELEASECOATING

A biofouling release coating prepared as in Example 4 was extracted intoluene as described in Example 2.

6. REGENERATION OF ADDITIVE-ENHANCED BIOFOULING RELEASE COATING

An extracted biofouling release coating was prepared as in Example 5,then soaked in a bath of approximately 300 mlpolydimethyidiphenylsiloxane oil (Mn about 2000) for 3 days, and wipeddry.

7. DETERMINATION OF BIOFOULING RELEASE PROPERTIES

Samples prepared according to each of the examples 1-6 were placed inthe Indian River in Florida in cages until barnacle settlement occurred.The barnacle attachment strength was measured on two identicallyprepared panels for each surface coating, according to the barnacleadhesion test method as described below. Results are shown in Table 1.The data indicate that the enhanced biofouling release properties of anoil-depleted sample is restored by exposure to a new additive source.

Biofouling release properties were quantitated by a standard barnacleadhesion test. This test measures the force required to remove barnaclesfrom a surface and provides data that can be used to compare the abilityof surfaces to reduce biofouling (barnacle) adhesion under fieldconditions as described in Example 7. This test was approved as an ASTMstandard in 1994 (ASTM D5618, 1994).

Live barnacles selected for testing were between 5 and 20 millimeters(mm) diameter at the base adhering to the surface to be tested. Theirspecies and condition were noted. Barnacles were preferably at least 20mm from the edge of the test substrate panel, attached to undamagedareas of the test surface, and not in direct contact with otherbarnacles. The barnacle base was measured with calipers in fourdirections (0, 45, 90, 135 degrees) to obtain an average base diameter,taking care not to dislodge the barnacle. The barnacle base plate area,A, was then estimated using the average base diameter, d_(a), accordingto the formula

    A=(πd.sub.a.sup.2)/4.

A shear force was then applied to the barnacle base, parallel to thetesting surface using a hand-held Shimpo mechanical dial force gauge(range=0-89 N (0-20 lb); accuracy=±0.3%; resolution=0.45N (0.1 lb)) at arate of approximately 4.15N s⁻¹ (1.0 lb s⁻¹) until the barnacle becamedetached or the maximum shear force was reached. The force (F) requiredfor detachment was noted. If more than 10% of the barnacle base platewas left attached to the substrate, the test was deemed void sinceincomplete removal of the barnacle base plate from the surface duringtesting may lead to erroneous adhesion values. The strength of adhesion,τ, was then calculated according to the formula

    τ=F/A.

Whenever possible, a minimum of ten measurements were made per coating.

                  TABLE 1                                                         ______________________________________                                        Barnacle Adhesion Values                                                          Example              Barnacle Adhesion (psi)                              ______________________________________                                        Ex. 1: release coating manufactured                                                                panel 1 13.95 +/- 4.27                                     without incorporation of additives panel 2 14.07 +/- 4.01                     Ex. 2: release coating manufactured panel 1 12.63 +/- 3.36                    without incorporation of additives panel 2 16.38 +/- 5.10                     (extracted)                                                                   Ex. 3: release coating manufactured panel 1 5.43 +/- 2.24                     without incorporation of additives panel 2 7.75 +/- 6.19                      (restored)                                                                    Ex. 4: release coating manufactured panel 1 8.29 +/- 2.16                     with incorporation of additives panel 2 0.29 +/- 0.24                         Ex. 5: release coating manufactured panel 1 14.96 +/- 3.79                    with incorporation of additives panel 2 15.39 +/- 3.68                        (extracted)                                                                   Ex. 6: release coating manufactured panel 1 10.81 +/- 4.21                    with incorporation of additives panel 2 9.91 +/- 3.88                         (restored)                                                                  ______________________________________                                    

The tests provide data for surfaces that have been coated with abiofouling release coating manufactured with or without an additive toincrease the release properties of the coating (examples 1 and 4), andthose same coated surfaces after an oil depletion treatment (examples 2and 5), and after subsequent restoration with the inventive method(examples 3 and 6).

The data confirm that experimental oil-depletion of biofouling releasecoatings manufactured with oil reduces the effectiveness of the releasecoating (compare examples 4 and 5), as does the oil-depletion known tooccur with aging. Coatings lacking the additive were unaffected by thesimulated aging, as expected (compare examples 1 and 2).

Use of the inventive method for restoring enhanced biofouling releaseproperties to such a coating decreases the amount of force needed toremove the barnacles, evidencing a restoration of the enhanced releaseproperties (compare examples 5 and 6). The inventive method alsoimproves the biofouling release properties of coatings not originallymanufactured with oils (compare examples 1 and 2 with example 3). Theresults in Table 1 demonstrate clearly that intact silicone biofoulingrelease coatings are capable of capturing or recapturing additivesapplied to their surfaces, and acquiring or reacquiring the enhancedbiofouling release properties those additives impart.

What is claimed is:
 1. A method of introducing enhanced biofoulingrelease properties to an intact, oil-depleted biofouling releasecoating, which method comprises exposing the surface of the biofoulingrelease coating to a restorative compound for a time sufficient toeffect enhancement of biofouling release properties, wherein therestorative compound is selected from the group consisting of siliconeoil, a hydrocarbon oil, a grease, a wax, and combinations thereof. 2.The method of claim 1, wherein the biofouling release coating comprisesa silicone-based polymer.
 3. The method of claim 2, wherein the polymercomprises a polydialkylsiloxane.
 4. The method of claim 3, wherein thepolydialkylsiloxane has the formula: ##STR3## wherein each R¹ is ahydroxyl radical or ##STR4## each R² is independently a hydrocarbon orfluorinated hydrocarbon radical, each R³ and R⁴ is a hydrocarbonradical, a is 0 or 1, and m has a value such that the viscosity of saidcompound under ambient temperature and pressure conditions is up toabout 50,000 centipoise.
 5. A method according to claim 1, wherein therestorative compound is a silicone oil.
 6. A method according to claim5, wherein the silicone oil is a polymethylphenylsiloxane oil.
 7. Amethod according to claim 1, wherein the time of exposure is at leastabout 10 hours.
 8. A method according to claim 7, wherein the time ofexposure is from about 10 to about 90 hours.
 9. A method according toclaim 8, wherein the time of exposure is from about 24 to about 72hours.
 10. A method according to claim 1, wherein the biofouling releasecoating was manufactured without enhancing additives.
 11. A methodaccording to claim 1, wherein the biofouling release coating wasmanufactured with enhancing additives.